The present invention relates to electromagnetic drivers for oscillating-conduit Coriolis-type mass flowmeters.
In response to the need to measure the quantity of material being delivered through pipelines, numerous types of flowmeters have evolved from a variety of design principles. One of the more widely used types of flowmeters is based on volumetric flow. Volumetric flowmeters are at best inaccurate in determining the quantity of material delivered, where the density of the material varies with temperature of feedstock or where the fluid being pumped through the pipe line is polyphase such as a slurry or where the fluid is non-Newtonian such as mayonnaise and other food products. In addition, chemical reactions, which are in effect mass reactions where proportions of reactants are critical, may be poorly served by volumetric flowmeters.
A mass flowmeter, on the other hand, is an instrument that provides a direct indication of the quantity of mass, as opposed to volume, of material being transferred through the pipeline. Various methods for measuring mass flow in a moving stream require application of a force to the stream and detecting and measuring some consequence of the applied force.
One class of mass measuring flowmeters is based on the well-known Coriolis effect. An exemplary Coriolis-type mass flowmeter is described in a co-pending U.S. patent application Ser. No. 923,847 filed Oct. 28, 1986, now U.S. Pat. No. 4,891,991 by Mattar et al., entitled "Coriolis-Type Mass Flowmeter" assigned to the assignee of the present invention and incorporated herein by reference in its entirety.
Many Coriolis-type mass flowmeters induce a Coriolis force by oscillating the pipe sinusoidally about a pivot axis orthogonal to the length of the pipe. In such a mass flowmeter, Coriolis forces are exhibited in the radial movement of mass in a rotating conduit. Material flowing through the pipe becomes a radially travelling mass which, therefore, experiences an acceleration. The Coriolis reaction force experienced by the travelling fluid mass is transferred to the pipe itself and is manifested as a deflection or offset of the pipe in the direction of the Coriolis force vector in the plane of rotation.
A major difficulty in these oscillatory systems is that the Coriolis force and, therefore, the resulting deflection is relatively small compared not only to the drive force but even to extraneous vibrations. On the other hand, an oscillatory system can employ the inherent bending resiliency of the pipe itself as a hinge or pivot point for oscillation that obviates the need for separate rotary or flexible joints, which improves mechanical reliability and durability. Moreover, an oscillatory system offers the possibility of using the resonant frequency of vibration of the tube itself to reduce the drive energy needed.
Energy is supplied to the tubes by a driving mechanism that oscillates the tubes by applying a periodic force. A typical type of driving mechanism is exemplified by an electromechanical driver, which exhibits motion proportional to a voltage applied across its coil. In an oscillating flowmeter the applied voltage is periodic and, generally, is sinusoidal. As mentioned above, the period of the input voltage, and hence, the driving force, is chosen to match the resonant frequency of the tube to reduce the energy needed to sustain the oscillation.
The Coriolis force resulting from the oscillation and the mass flow within the tube is measured by sensors also disposed on the flowmeter tube. In some cases it is desirable to place the sensors in close proximity to the driving mechanism. For example, in some systems this arrangement results in a more accurate determination of the Coriolis force exhibited by the flowmeter tube.